A fault protection or detection unit may be employed for monitoring one or several transmission lines of a power transmission system, or network, or grid. In the event of a fault occurring in a transmission line, the fault protection unit may trip, or open, circuit breakers located at each end of the transmission line in order to disconnect the transmission line in which a fault occurs from the rest of the power transmission system. Thereby, the risk of propagation of the fault in the power transmission system may be mitigated or even eliminated. Subsequently, depending on the types of fault, the circuit breakers may be automatically closed in order to quickly restore power transmission via the transmission line that was previously disconnected from rest of the power transmission system. This may be referred to as auto-reclosing. Hence, once the temporary or transient fault has been cleared, or after some selected or predefined period of time based on arc extinction time, which for example may be set based on the estimated time required to quench arcs which may appear in the circuit breakers when they are opened to interrupt the current. Auto-reclosing may for example be initiated on a condition that a trip signal is transmitted to the circuit breaker(s).
Thus, in overhead transmission lines or overhead lines (OHLs) auto-reclosing protection may be utilized. In OHLs, most of the faults are temporary or transient faults, for example due to a lightning strike and/or flashover. Due to the temporary or transient nature of the faults, an auto-reclosing protection scheme may therefore be applied. On the other hand, in transmission lines which are based solely on cables, virtually all of the faults that may occur are permanent faults, which for example may be due to a short-circuit between conductors. In that case, auto-reclosing should not be applied. In some cases, permanent faults may occur also in OHLs. In the event of a permanent fault, the faulty line will typically have to be tripped (isolated) again by the protection equipment after unsuccessful auto-reclosing actions. Then, the line with permanent fault will be serviced by maintenance personnel.
In High Voltage Direct Current (HVDC) power systems, DC circuit breakers may be tripped following a DC fault in a HVDC transmission line, and after that the HVDC transmission line is often re-energized by reclosing the DC circuit breakers. If the fault is temporary, the HVDC converters will ramp up their power outputs back to the desired HVDC transmission line load. If the fault is permanent, the HVDC transmission line will be tripped again and then de-energized for maintenance. Consider as an example a multi-terminal voltage source converter (VSC) based HVDC power system. In such a power system, faults on a DC cable or a DC OHL are typically isolated from the rest of the power system by temporarily shutting down the DC line, or temporarily taking the DC line out of operation, using DC circuit breakers. For an OHL, the DC line may be put back into operation after a selected or predefined reclosing waiting time based on arc extinction time to allow for the insulation to be restored in case it is a temporary fault. However, if the fault is a permanent fault, the DC line will be shut down again after reclosing it. In that case, the DC line may have to be shut down for a relatively long period, in order to allow for maintenance personnel to locate and clear the fault. Events leading to DC line faults could happen mainly during normal operation when the DC line is energized. For faults in DC OHLs, DC circuit breakers are typically designed to auto-reclose after some selected or predefined time period, with the length of the time period being based on the arc extinction time that it takes for the insulation in the fault location to be restored. In case of temporary faults, the DC circuit breakers will first be tripped, and the DC line will then be restored back to normal operation following successful auto reclosing. In case of permanent faults however, the DC circuit breakers need to be tripped again after auto reclosing since there may still be a fault present in the DC line.
In HVDC grids, only the component(s) or element(s) of the power system in which there is a fault should preferably be isolated from the rest of the power system, so as to ensure continued operation of the HVDC grid and reduce or minimize downtime thereof. Isolation of the faulty component(s) or element(s) is usually achieved by way of operation of circuit breakers. Reclosing a circuit breaker relatively quickly following opening of a circuit breaker is often done in the interest of maintaining stability of the electric power system and quick power restoration. For example HVDC grids which are used for transmission, distribution, etc., of power generated based on renewable energy sources are expected to include transmission lines containing different types of transmission line portions or sections, e.g., so as to include cable and overhead line sections. Such transmission lines may in the following be referred to as mixed transmission lines, or mixed configuration transmission lines. One application where such transmission lines are expected to be increasingly utilized is in transmission or distribution of power generated at offshore wind power generating plants to onshore locations.